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How come the surface area is different
If the evaporator size has not changed in at least 6 years. And I understand the work that it has to be done for the retrofit including line flush and txv replacement.

Surface area is larger due to increased seer rating requirements by uncle Sam and company... R-22 coils were max 350# I believe and the 410a is somewhwere in the neighborhood of 5-600# . If space is the issue and not cost they do make some lower profile coils. There would be n obig issue you couldn't overcome (txv,flush,saturation points etc.) Other thatn the test pressure and matching seer rating.

I would never try to flush the evap coil. You just can't get all of the oil out of the coil. Replace the evap coil. Armstrong Air will not honor the warranty if you do not replace the evap coil when you change to a 410A condenser.

If you insist on reusing the coil, I'd skip the flush. You have that oil diluter left in the coil. NO WAY of blowing it all out of the coil like there is a lineset since coils have multiple passes. Nobody making uncharged units that big? Rheem stops at 10 ton.